More than 70 percent of the earth's surface is covered by water. However, most of the water is seawater, which cannot be drunk as its salt content is too high. Therefore, freshwater or drinking water is very scarce. Desalinating or purifying the seawater is a solution for getting freshwater or drinking water. Generally, the seawater is desalinated by chemical subtraction, electrically-powered distillation, or solar-powered distillation. Chemical subtraction and electrically-powered distillation are prohibitively expensive due to the volume of material and power required. Therefore solar-powered distillation is the method most commonly used in desalinating seawater.
Solar-powered distillation operates by solar-powered distillation evaporating seawater using solar energy. As the water evaporates it leaves behind its salt content, the vapor can then be cooled, for example by using a condenser, producing purified water that can be drunk or used in agriculture. A typical solar-powered distillation method for desalinating seawater uses a large greenhouse-like structure having a transparent walls and roof. The solar energy transmits the transparent walls, and heats seawater contained in the green house. The seawater evaporates, and the vapor is cooled to form a supply of purified water. However, the large greenhouse uses too much space and has a low heating efficiency.
China patent No. 01137342.3 discloses an apparatus for desalinating seawater using evacuated solar collector tubes. The seawater is fed into the collector tubes, solar energy is then concentrated on the collector tubes and the seawater in the collector tubes is heated to vapor. This vapor is then cooled to form pure water. This apparatus has good heating efficiency, but the salt in the seawater is deposited on the inner surface of the collector tubes, decreasing the heating efficiency of the collector tubes, and shortening the useful life-time of the collector tubes.
Therefore, a water purification apparatus which can overcome the above-described problems is desired.